Routing quantum signals in the microwave domain using time dependent switching

ABSTRACT

A technique relates to configuring a superconducting router. The superconducting router is operated in a first mode. Ports are configured to be in reflection in the first mode in order to reflect a signal. The superconducting router is operated in a second mode. A given pair of the ports is connected together and in transmission in the second mode, such that the signal is permitted to pass between the given pair of the ports.

BACKGROUND

The present invention relates to superconducting electronic devices, andmore specifically, circulation/routing of quantum signals in themicrowave domain using time dependent switching.

A radio frequency (RF) and microwave switch is a device to route highfrequency signals through transmission paths. RF and microwave switchesare used extensively in microwave test systems for signal routingbetween instruments and devices. Incorporating a switch into a switchmatrix system enables one to route signals from multiple instruments tosingle or multiple devices. Similar to electrical switches, RF andmicrowave switches come in different configurations providing theflexibility to create complex matrixes and automated test systems formany different applications.

In physics and computer science, quantum information is information thatis held in the state of a quantum system. Quantum information is thebasic entity of study in quantum information theory, and can bemanipulated using engineering techniques known as quantum informationprocessing. Much like classical information can be processed withdigital computers, transmitted from place to place, manipulated withalgorithms, and analyzed with the mathematics of computer science,analogous concepts apply to quantum information. Quantum systems such assuperconducting qubits are very sensitive to electromagnetic noise, inparticular in the microwave and infrared domains.

SUMMARY

According to one or more embodiments, a method of configuring asuperconducting router is provided. The method includes operating thesuperconducting router in a first mode, in which ports are configured tobe in reflection in the first mode in order to reflect a signal. Also,the method includes operating the superconducting router in a secondmode, in which a given pair of the ports is connected together and intransmission in the second mode, such that the signal is permitted topass between the given pair of the ports.

According to one or more embodiments, a method of configuring asuperconducting circulator is provided. The method includes operatingthe superconducting circulator to receive a readout signal at an inputport while an output port is in reflection. The readout signal is to betransmitted through a common port to a quantum system, and the readoutsignal is configured to cause a reflected readout signal to resonateback from the quantum system. Also, the method includes operating thesuperconducting circulator to output the reflected readout signal at theoutput port while the input port is in reflection.

According to one or more embodiments, a superconducting router isprovided. The superconducting router includes ports configured tooperate in a first mode and a second mode. In the first mode, the portsare configured to be in reflection in order to reflect a signal. Thesuperconducting router includes a given pair of the ports configured tooperate in the second mode. In the second mode, the given pair of theports is connected together and in transmission, such that the signal ispermitted to pass between the given pair of the ports.

According to one or more embodiments, a superconducting circulator isprovided. The superconducting circulator includes an input portconnected to a first tunable filter such that the input port isconfigured to operate in a first mode and a second mode, and an outputport connected to a second tunable filter such that the output port isconfigured to operate in the first mode and the second mode. In thefirst mode, the input port is configured to receive a readout signalwhile the output port is in reflection, and the readout signal is to betransmitted through a common port to a quantum system. The readoutsignal is configured to cause a reflected readout signal to resonateback from the quantum system. In the second mode, the output port isconfigured to output the reflected readout signal while the input portis in reflection.

According to one or more embodiments, a system is provided. The systemincludes a quantum system, and a superconducting microwave switchconnected to the quantum system. The superconducting microwave switch isconfigured to receive a readout signal at an input port, and the readoutsignal is to be transmitted through a common port to the quantum system.The superconducting microwave switch is configured to output a reflectedreadout signal at an output port, and the reflected readout signal isfrom the quantum system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a superconducting microwave switch/routeraccording to one or more embodiments.

FIG. 2 is a block diagram of the superconducting microwave switch/routerin FIG. 1 according to one or more embodiments.

FIG. 3 is a schematic of the superconducting microwave switch/routerillustrating transmission as the mode of operation according to one ormore embodiments.

FIG. 4 is a schematic of the superconducting microwave switch/routerillustrating reflection as the mode of operation according to one ormore embodiments.

FIG. 5 is a schematic of a superconducting microwave switch/routeraccording to one or more embodiments.

FIG. 6 is a block diagram of the superconducting microwave switch/routerin FIG. 5 according to one or more embodiments.

FIG. 7 is a schematic of a superconducting microwave switch/routeraccording to one or more embodiments.

FIG. 8 is a schematic of a superconducting microwave switch/routeraccording to one or more embodiments.

FIG. 9 is a schematic of an N port superconducting microwaveswitch/router according to one or more embodiments.

FIG. 10 is a flow chart of a method of configuring a superconductingmicrowave switch/router according to one or more embodiments.

FIG. 11 is a flow chart of a method for configuring a superconductingmicrowave switch/router according to one or more embodiments.

FIG. 12 is a flow chart of a method of configuring a superconductingmicrowave switch/router according to one or more embodiments.

FIG. 13 is a flow chart of a method of configuring a superconductingmicrowave switch/router according to one or more embodiments.

FIG. 14 is a simplified view of the superconducting microwaveswitch/router can connect between arbitrary pairs of nodes according toone or more embodiments.

FIG. 15A depicts the superconducting microwave switch/router with alltunable filters in reflection according to one or more embodiments.

FIG. 15B depicts the superconducting microwave switch/router in aconfiguration that permits transmission between two ports according toone or more examples.

FIG. 15C depicts the superconducting microwave switch/router in aconfiguration that permits transmission between two ports according toone or more examples.

FIG. 15D depicts the superconducting microwave switch/router in aconfiguration that permits transmission between two ports according toone or more examples.

FIG. 16A depicts an example scenario at a first timing according to oneor more embodiments.

FIG. 16B depicts an example scenario at a second timing according to oneor more embodiments.

FIG. 16C depicts an example scenario at third timing according to one ormore embodiments.

FIG. 16D depicts an example scenario at fourth timing according to oneor more embodiments.

FIG. 17 is a flow chart of a method of configuring a superconductingrouter according to one or more embodiments.

FIG. 18 is a flow chart of a method of configuring a superconductingcirculator according to one or more embodiments.

DETAILED DESCRIPTION

Various embodiments are described herein with reference to the relateddrawings. Alternative embodiments can be devised without departing fromthe scope of this document. It is noted that various connections andpositional relationships (e.g., over, below, adjacent, etc.) are setforth between elements in the following description and in the drawings.These connections and/or positional relationships, unless specifiedotherwise, can be direct or indirect, and are not intended to belimiting in this respect. Accordingly, a coupling of entities can referto either a direct or an indirect coupling, and a positionalrelationship between entities can be a direct or indirect positionalrelationship. As an example of an indirect positional relationship,references to forming layer “A” over layer “B” include situations inwhich one or more intermediate layers (e.g., layer “C”) is between layer“A” and layer “B” as long as the relevant characteristics andfunctionalities of layer “A” and layer “B” are not substantially changedby the intermediate layer(s).

In accordance with one or more embodiments, superconducting (orlossless) microwave switches/routers allow one to route quantum signalson demand between different nodes of a circuit or between differentports. Superconducting microwave switches can have many applications inthe area of quantum information processing. For example, superconductingmicrowave switches can be utilized for time-multiplexed readout,time-multiplexed driving (e.g., cross-resonance drives),time-multiplexed characterization of several devices, time-multiplexedinteraction between pairs of quantum systems, time-dependent circulationof signals, etc.

According to one or more embodiments, a superconducting microwave switchthat can have one input port and N output ports is provided. Also, thesuperconducting microwave switch can have one output port and N inputports. Each of the ports of the superconducting microwave device isdesigned to have the same characteristic impedance Z₀. In oneimplementation, each input-output pair is connected through a tunablelow-pass filter whose cutoff frequency can be tuned in-situ usingapplied magnetic flux. The tunable low-pass filter can be implementedusing a ladder of series inductive elements (e.g., direct current (DC)superconducting quantum interference devices (SQUIDs)) and shuntcapacitive elements (e.g., lumped-element capacitors). In anotherimplementation, each input-output pair can be connected through atunable high-pass filter whose cutoff frequency can be tuned in-situusing applied magnetic flux, and the tunable high-pass filter can beimplemented using series capacitive elements (e.g., lumped-elementcapacitors) and shunt inductive elements (e.g., DC-SQUIDs).

Now turning to the figures, FIG. 1 is a schematic of a superconductingmicrowave switch/router 100 according to one or more embodiments. FIG. 1illustrates building blocks of the superconducting microwaveswitch/router 100 based on a tunable filter 20. In this example, thetunable filter 20 is a tunable low-pass filter (TLPF).

In this example, the microwave switch/router 100 includes ports 10, suchas for example, ports 1 and 2. The ports 10 are input and output ports.The tunable filter 20 includes one or more unit cells 60. Each unit cell60 includes a variable inductor 40 designated as variable inductiveelement L₁ (and other examples include L2, L3, and DC-SQUIDs discussedfurther below), and each unit cell 60 includes a capacitor 50 designatedas capacitive element C. In each unit cell 60, the variable inductor L₁40 is connected in series with ports 10, and a capacitor C 50 isconnected to one end of the variable inductor 40 and to ground. Therecan be N number of unit cells 60 repeated and connected together (inseries) in the tunable filter 20 for a total of N unit cells. For N unitcells, the inductors L1 40 are connected in series, with each inductorL1 40 shunted to ground by its respective capacitor 50. Theinterconnection of the ports 10, variable inductors L1 40, andcapacitors C 50 is by transmission line 30. The transmission line 30acts as a superconducting wire or waveguide to carry a microwave signalfrom port 1 via the tunable filter 20 to port 2, or vice versa. Acoaxial cable can connect to the external ends of the ports 10 such thatone coaxial cable inputs microwave signals and another coaxial cableoutputs the microwave signals. The transmission line 30 can be astripline, microstrip, etc. The variable inductors 40, capacitors 50,and transmission lines 30 are made of superconducting material. Examplesof superconducting materials (at low temperatures, such as about 10-100millikelvin (mK), or about 4 K) include niobium, aluminum, tantalum,etc.

FIG. 2 is a block diagram of the superconducting microwave switch/router100 in FIG. 1 according to one or more embodiments. FIG. 2 is anequivalent circuit to FIG. 1 without depicting the internal details ofthe tunable filter 20.

It can be assumed that the microwave signal that is to be transmittedthrough the superconducting microwave switch/router 100 has a centerangular frequency ω₀. The impedance designation Z₀ is the characteristicimpedance at ports 1 and 2 (which can be the input and output ports orvice versa). For example, the characteristic impedance Z₀ can be 50 ohms(Ω) at each port.

For an individual unit cell 60, the impedance is Z₁ where Z₁=√{squareroot over (^(L) ₁/C)} and where the angular frequency ω₁ of the unitcell 60 is ω₁₌ ¹/√{square root over (CL₁)}. The cutoff angular frequencyof the tunable filter 20 denoted as ω_(C) is on the order of the angularresonance frequency ω₁ of the unit cell 60 (or multiple unit cells addedtogether) and it is correlated with ω₁, meaning ω_(C) increases anddecreases with ω₁. The exact dependence of ω_(C) on ω₁ and on the numberof unit cells N can be found through a microwave simulation orcalculation. From this it follows that the cutoff frequency ω_(C) of thetunable filter 20 depends on the values of the variable inductor L₁ 40and the capacitor C 50 (for the one or more unit cells 60). Inparticular, the inductance of the variable inductor L₁ 40 controls thecutoff frequency ω_(C) of the tunable filter 20, thereby controllingwhen the tunable filter 20 is operating in transmission or reflectionwith respect to the microwave signal (center angular frequency ω₀). Theinductance of the variable inductors L₁ 40 has an inverse relationshipto the cutoff frequency ω_(C). For example, when the inductance of thevariable inductor L₁ 40 is increased, the cutoff frequency ω_(C) of thetunable filter 20 is decreased. Conversely, when the inductance of thevariable inductor L₁ 40 is decreased, the cutoff frequency ω_(C) of thetunable filter 20 is increased. It is noted that varying the inductanceof the unit cells does not only change the cutoff frequency of thefilter but also changes its characteristic impedance. Therefore, it canbe desirable that Z₁ or the characteristic impedance of the filtermatches the characteristic impedance of the ports as much as possiblewhen the switch is closed, i.e., operated in the transmission mode.

Accordingly, when operating as a closed switch, the superconductingmicrowave switch/router 100 is controlled to pass the microwave signal(center angular frequency ω₀) in transmission from port 1 to port 2 (orvice versa) by decreasing the inductance of the variable inductor L₁ 40in the tunable filter 20. This allows the microwave signal (centerangular frequency ω₀) to fall within the low-pass band of the tunablefilter 20. When operating as an open switch, the superconductingmicrowave switch/router 100 is controlled to block transmission of themicrowave signal (center angular frequency ω₀) from port 1 to port 2 (orvice versa) using reflection by increasing the inductance of thevariable inductor L₁ 40 in the tunable filter 20. This allows themicrowave signal (center angular frequency ω₀) to fall outside of thelow-pass band and thus be attenuated or in other words reflected.

To further explain operation of the modes of operation for thesuperconducting microwave switch/router 100, FIGS. 3 and 4 arediscussed. FIG. 3 is a schematic of the superconducting microwaveswitch/router 100 illustrating transmission as the mode of operationaccording to one or more embodiments. In FIG. 3, the tunable filter 20is tuned such that the center angular frequency ω₀ of the incomingmicrowave signal 305 through the device port is less than the cutofffrequency ω_(C) of the tunable filter 20, i.e., ω₀<ω_(C). In this modeof operation, the tunable filter 20 is configured to operate intransmission because the frequency of the microwave signal 305 is lessthan the cutoff frequency of the tunable low-pass filter 20. Under thiscondition, the microwave signal 305 is transmitted from port 1 throughthe tunable filter 20 to port 2, such that the microwave signal 305 isoutput as desired.

FIG. 4 is a schematic of the superconducting microwave switch/router 100illustrating reflection as the mode of operation according to one ormore embodiments. In FIG. 4, the tunable filter 20 is tuned such thatthe center angular frequency ω₀ of the microwave signal 305 is greaterthan the cutoff frequency ω_(C) of the tunable filter 20, i.e.,ω₀>ω_(C). In this mode of operation, the tunable filter 20 is configuredto operate in reflection because the frequency of the microwave signal305 is greater than the cutoff frequency of the tunable low-pass filter20. Under this condition, when the microwave signal 305 enters throughport 1, the microwave signal 305 is blocked from passing to port 2because the tunable filter 20 reflects the microwave signal 305, therebynot allowing the microwave signal 305 to pass from port 1 to port 2.

FIG. 5 is a schematic of a superconducting microwave switch/router 100according to one or more embodiments. FIG. 6 is a block diagram of thesuperconducting microwave switch/router 100 in FIG. 5 according to oneor more embodiments. FIG. 6 is an equivalent circuit to FIG. 5 withoutdepicting the internal details of the tunable filter 20. FIGS. 5 and 6are analogous to FIGS. 1 and 2, except that FIGS. 5 and 6 have beenextended to 3 ports instead of 2 ports. It is understood that thesuperconducting microwave switch/router 100 can be extended to N numberof ports as desired according to embodiments.

In the configuration depicted in FIGS. 5 and 6, there are two tunablefilters 20. One tunable filter 20 is connected between port 1 and port2, while the other tunable filter 20 is connected between port 1 andport 3. Each of the tunable filters 20 is formed of one or more unitcells 60 as discussed above. For explanation purposes, the one or morevariable inductors 40 are identified as L₂ in the tunable filter 20connected between ports 1 and 2, while the one or more variableinductors 40 are identified as L₃ in the tunable filter 20 connectedbetween ports 1 and 3. The tunable filters 20 between ports 1 and 2 andports 1 and 3, respectively, are individually controlled such that onecan be in transmission while the other is operating in reflection.

The tunable filter 20 between ports 1 and 2 includes one or more unitcells 60. Each unit cell 60 includes a variable inductor L₂ 40 andcapacitor 50. In each unit cell 60, the variable inductor L₂ 40 isconnected in series with ports 1 and 2, and the capacitor C 50 isconnected to one end of the variable inductor 40 and to ground. Therecan be N number of unit cells 60 repeated and connected together in thetunable filter 20 for a total of N unit cells between ports 1 and 2. Forthe tunable filter 20 between ports 1 and 2, the impedance of each unitcell is Z₂ where Z₂=√{square root over (^(L) ₂/C)} and the angularfrequency is ω₂ where ω₂=1/√{square root over (CL₂)}.

Similarly, the tunable filter 20 connected between ports 1 and 3includes one or more unit cells 60. Each unit cell 60 includes avariable inductor L₃ 40 and capacitor 50. In each unit cell 60, thevariable inductor L₃ 40 is connected in series with ports 1 and 3, andthe capacitor C 50 is connected to one end of the variable inductor L₃40 and to ground. There can be N number of unit cells 60 repeated andconnected together in the tunable filter 20 for a total of N unit cellsbetween ports 1 and 3. For the tunable filter 20 connected between ports1 and 3, the impedance of each unit cell is Z₃ where Z₃=√{square rootover (^(L) ₃/C)} and the angular frequency is ω₃ where ω₃=√{square rootover (CL₃)}.

It should be appreciated that additional ports and tunable filters canbe analogously added as desired.

In FIG. 2, the cutoff frequency of the single tunable filter 20 wasdesignated as ω_(C) above. Because more than one tunable filter 20 isprovided in FIGS. 5 and 6, the tunable filter 20 connected between ports1 and 2 is designated as cutoff frequency ω_(C2) while the tunablefilter 20 connected between ports 1 and 3 is designated as cutofffrequency ω_(C3).

For operation of the microwave signal 305 in transmission from/betweenport 1 to port 2 (or vice versa), the tunable filter 20 between ports 1and 2 is tuned such that the center angular frequency ω₀ of themicrowave signal 305 is less than the cutoff frequency ω_(C2) of thetunable filter 20 between ports 1 and 2, while the tunable filter 20between ports 1 and 3 is tuned such that the center angular frequency ω₀of the microwave signal 305 is much greater than the cutoff frequencyω_(C3) between ports 1 and 3: ω_(C3)<<ω₀<ω_(C2). In this mode ofoperation, the tunable filter 20 between ports 1 and 2 is configured tooperate in transmission because the microwave signal 305 (ω₀) is lessthan the cutoff frequency ω_(C2), and therefore, the microwave signal305 is transmitted from port 1 through the tunable filter 20 to port 2,such that the microwave signal 305 is output as desired. Concurrently,the tunable filter 20 connected between ports 1 and 3 is configured tooperate in reflection because the microwave signal 305 (ω₀) is greaterthan the cutoff frequency (ω_(C3)), and therefore, the microwave signal305 is blocked from passing between ports 1 and 3. Additional conditionsfor transmission from port 1 to port 2 (or vice versa) include Z₂ ˜ Z₀for impedance matching. Additional conditions for reflectionfrom/between ports 1 and 3 include Z₃>>Z₀.

On the other hand, for operation of the microwave signal 305 intransmission from/between port 1 to port 3 (or vice versa), the tunablefilter 20 between ports 1 and 3 is tuned such that the center angularfrequency ω₀ of the microwave signal 305 is less than the cutofffrequency ω_(C3) of the tunable filter 20 between ports 1 and 3, whilethe tunable filter 20 between ports 1 and 2 is tuned such that thecenter angular frequency ω₀ of the microwave signal 305 is much greaterthan the cutoff frequency ω_(C2) between ports 1 and 2:ω_(C2)<<ω₀<ω_(C3). In this mode of operation, the tunable filter 20between ports 1 and 3 is configured to operate in transmission becausethe microwave signal 305 (ω₀) is less than the cutoff frequency ω_(C3),and therefore, the microwave signal 305 is transmitted from port 1through the tunable filter 20 to port 3, such that the microwave signal305 is output as desired. Concurrently, the tunable filter 20 connectedbetween ports 1 and 2 is configured to operate in reflection because themicrowave signal 305 (ω₀) is greater than the cutoff frequency (ω_(C2)),and therefore, the microwave signal 305 is blocked from passing betweenports 1 and 2 in this example. Additional conditions for transmissionfrom port 1 to port 3 (or vice versa) include Z₃ ˜ Z₀ for impedancematching. Additional conditions for reflection from/between ports 1 and2 include Z₂>>Z₀.

FIG. 7 is a schematic of a superconducting microwave switch/router 100according to one or more embodiments. FIG. 7 is analogous to FIGS. 5 and6, except that FIG. 7 implements the lossless/superconducting microwaveswitch/router 100 utilizing direct current (DC) superconducting quantuminterference devices (SQUIDs). In FIG. 7, each of the variable inductors40 (discussed above) is implemented as (variable) DC-SQUIDs 705 in thetunable filter 20. It is noted that the tunable filters 20 in FIG. 7 areconfigured to operate in transmission and reflection with respect toeach of the tunable filters 20 as discussed above. Also, it isunderstood that the superconducting microwave switch/router 100 can beextended to N number of ports as desired according to embodiments.

In the configuration depicted in FIG. 7, there are two tunable filters20 and three ports 10 depicted although more ports 10 and tunablefilters 20 can be analogously added. One tunable filter 20 is connectedbetween port 1 and port 2, while the other tunable filter 20 isconnected between port 1 and port 3. Each of the tunable filters 20 isformed of one or more unit cells 60 as discussed herein.

For the tunable filter 20 connected between port 1 and port 2, each unitcell 60 includes one or more DC-SQUIDs 705_2. In the unit cell 60, thecapacitor C 50 connects/shunts the one or more DC-SQUIDs 705_2 toground. When more than one DC-SQUID 705_2 is utilized in the unit cell60, the DC-SQUIDs 705_2 are connected together in series. There can be atotal of M DC-SQUIDs 705_2 per unit cell, where M is an integer of 1 ormore. The tunable filter 20 between ports 1 and 2 includes one or moreunit cells 60, such that each unit cells 60 is connected in series withports 1 and 2, and the capacitor C 50 is connected to one end of theDC-SQUID 705_2 and to ground. There can be N number of unit cells 60repeated and connected together in series in the tunable filter 20 for atotal of N unit cells between ports 1 and 2, where N is an integer of 1or more. For the tunable filter 20 between ports 1 and 2, the impedanceof each unit cell is Z₂ where Z₂=√{square root over (^(L) ₂/C)} and theangular frequency is ω₂ where ω₂=1/√{square root over (CL₂)}. It isnoted that each DC-SQUID 705_2 has an inductance and/or is an inductiveelement designated L₂.

For the tunable filter 20 connected between port 1 and port 3, each unitcell 60 includes one or more DC-SQUIDs 705_3. In the unit cell 60, thecapacitor C 50 connects/shunts the one or more DC-SQUIDs 705_3 toground. When more than one DC-SQUID 705_3 is utilized in the unit cell60, the DC-SQUIDs 705_3 are connected together in series.

There can be a total of M DC-SQUIDs 705_3 per unit cell, where M is aninteger of 1 or more. The tunable filter 20 between ports 1 and 3includes one or more unit cells 60, such that each unit cell 60 isconnected in series with ports 1 and 3, and the capacitor C 50 isconnected to one end of the DC-SQUID 705_3 and to ground. There can be Nnumber of unit cells 60 repeated and connected together in the tunablefilter 20 for a total of N unit cells between ports 1 and 3, where N isan integer of 1 or more. For the tunable filter 20 between ports 1 and3, the impedance of each unit cell is Z₃ where Z₃=√{square root over(^(L) ₃/C)} and the angular frequency is ω₃ where ω₃=1/√{square rootover (CL₃)}. It is noted that each DC-SQUID 705_3 has an inductanceand/or is an inductive element designated L₃.

Now, further information regarding DC-SQUIDs is provided below. A SQUID(Superconducting Quantum Interference Device) is a type ofsuperconducting electronic device well-known to those skilled in theart. In particular, the type of SQUID known as a DC-SQUID includes aloop formed of superconducting wire, superconducting thin-film metal orother superconducting material, interrupted by two or more Josephsonjunctions (JJ) 710. The SQUID contains two or more Josephson junctions710 in a current-carrying loop. As is widely understood by those skilledin the art, via the principle of quantum interference of superconductingcurrents, the combined Josephson critical current of the Josephsonjunctions within the SQUID will vary depending on the magnetic fluxthreading the SQUID loop. Likewise, the Josephson inductance exhibitedby the SQUID's Josephson junctions will also vary depending on suchmagnetic flux (which is magnetic flux Φ₂ for each DC-SQUID 705_2 andmagnetic flux Φ₃ for each DC-SQUID 705_3). Furthermore, arrays of SQUIDscan be arranged in an electrical circuit in such a way as to combinetheir inductances. It is specified that the magnetic flux of an in-planeloop represents a well-known and well-defined quantity including themagnetic field within the loop, multiplied by the cosine of the anglethat the field makes with the axis perpendicular to the loop, integratedacross the entire area of the loop. Thus, the SQUID is highly sensitiveto both the magnitude and the direction of the magnetic field in itsvicinity (for example, flux line 730_2 creates the magnetic field tothereby cause magnetic flux Φ₂ for each DC-SQUID 705_2, while flux line730_3 creates the magnetic field to thereby cause magnetic flux Φ₃ foreach DC-SQUID 705_3). The DC-SQUID 705_2 and 705_3 respectivelyexperience the magnetic flux Φ₂, magnetic flux Φ₃ from the respectivemagnetic fields created by flux line 730_2, flux line 730_3 and therebyits Josephson inductance (the Josephson inductance is designated L_(J2)for DC-SQUID 705_2 and L_(J3) for DC-SQUID 705_3) is changed. To oneskilled in the art, this sensitivity to magnetic field enables the SQUIDto be employed as a useful component in an electric circuit, in that thevariation of the SQUID's Josephson inductance causes useful changes inthe circuit's properties. The inductance L₂ and L₃ of the DC-SQUIDs705_2 and 705_3, respectively, corresponds to the Josephson inductanceL_(J2) for DC-SQUID 705_2 and L_(J3) for DC-SQUID 705_3. Toindependently change/control (increase or decrease) the inductance L₂and L₃ of the DC-SQUIDs 705_2 and 705_3, flux lines 730_2 and 730_3 areprovided. The flux lines are can be generally referred to as flux lines730. The flux lines 730_2 and 730_3 independently apply a magnetic‘bias’ field perpendicular to the SQUID loop of the respective DC-SQUIDs705_2 and 705_3, in order to set the ‘working point’ of the SQUID. Theflux line 730_2 has current I₂ that creates a magnetic field to causethe magnetic bias flux Φ₂ to change as desired. Similarly, the flux line730_3 has current I₃ that creates a magnetic field to cause the magneticbias flux Φ₃ to change as desired. Accordingly, the tunable filters 20between ports 1 and 2 and ports 1 and 3, respectively, are individuallycontrolled such that one can be in transmission while the other isoperating in reflection.

The inductance L₂ (per unit cell 60) for the tunable filter 20 betweenports 1 and 2 can be defined as L₂ ˜ ML_(J2)+L_(s), where M is thenumber of DC-SQUIDS 705_2 in a unit cell, where L_(J2) is the Josephsonjunction inductance of the DC-SQUID, and where L_(s) is the seriesinductance of the transmission lines 30 (wires) of each unit cell. Theinductance L₂ of each unit cell 60 is primarily based on the Josephsonjunction inductance L_(J2). Therefore, Josephson junction inductanceL_(J2) is defined below (without the series inductance L_(s) of thetransmission line 30 (wires)): the Josephson junction inductance

${L_{J\; 2} = \frac{L_{J\; 0}}{{\cos\left( {\pi\frac{\Phi_{2}}{\Phi_{0}}} \right)}}},$where L_(J0)=^(Φ) ⁰ /_(4πI) ₀ , where I₀ is the critical current of eachJosephson junction 710, wherein Φ₂ is the magnetic flux bias threadingthe loop, and where

$\Phi_{0} = \frac{h}{2e}$(superconducting magnetic flux quantum) in which h is Planck's constantand e is the electron charge.

Similarly, the inductance L₃ (per unit cell 60) for the tunable filter20 between ports 1 and 3 can be defined as L₃ ˜ ML_(J3)+L_(s), where Mis the number of DC-SQUIDS 705_3 in a unit cell, where L_(J3) is theJosephson junction inductance, and where L_(s) is the series inductanceof the transmission line 30 (wires) of each unit cell. The inductance L₃of each unit cell 60 is primarily based on the Josephson junctioninductance L_(J3). Therefore, Josephson junction inductance L_(J3) isdefined below (without the series inductance L_(s) of the transmissionline 30 (wires)): the Josephson junction inductance

${L_{J\; 3} = \frac{L_{J\; 0}}{{\cos\left( {\pi\frac{\Phi_{3}}{\Phi_{0}}} \right)}}},$where L_(J0)=^(Φ) ⁰ /_(4πI) ₀ , where I₀ is the critical current of the(two) Josephson junctions 710, where Φ₃ is the magnetic flux biasthreading the loop, and where

$\Phi_{0} = \frac{h}{2e}$(superconducting magnetic flux quantum) in which h is Planck's constantand e is the electron charge. In this analysis, the experimenters assumethat the DC-SQUIDs have small loops and that the self-inductance of theDC-SQUID loop is negligible compared to the Josephson inductance of theDC-SQUID.

It is noted that the inductance L₂ is the inductance of one unit cell 60out of N unit cells (N≥1) connected in series with the transmission linein the tunable filter 20 between ports 1 and 2, and likewise theinductance L₃ is the inductance of one unit cell 60 out of N unit cells(N≥1) connected in series with the transmission line in the tunablefilter 20 between ports 1 and 3.

It should be understood by one skilled in the art that the tunablefilter design discussed herein is not limited to identical unit cellswith respect to the inductive and capacitive elements in each unit cell.The identical unit cell picture is mainly presented here for simplicityand ease of understanding. In fact, varying the unit cells based on themicrowave filter theory can be advantageous and yield a betterperformance in terms of the maximum amplitude of ripples in the filterresponse, the filter flatness, the filter bandwidth, the amount ofreflection in-band and out-of-band, the amount of attenuation in thestopping band, etc. Accordingly, it should be appreciated that the unitcells may or may not be identical in one or more embodiments to employany or more of the advantages discussed above.

As should be recognized, the superconducting microwave switch/router 100can have one input port and N output ports in one configuration, and/orhave one output port and N input ports in another configuration. Allports 10 of the device have the same characteristic impedance Z₀. Eachinput-output pair is connected through a tunable low-pass filter whosecut-off frequency can be tuned in-situ using applied magnetic flux. Thetunable low-pass filter 20 can be implemented using a ladder ofinductive elements (DC-SQUIDs) and capacitive elements (lumped-elementcapacitors).

By controlling the DC currents I₂ and I₃ respectively applied to theflux lines 730_2 and 730_3, one can independently set the magnetic biasfluxes Φ₂ and Φ₃ which determine inductance L₂ and L₃ in each chain.This in turn tunes the cutoff angular frequencies ω_(C2), ω_(C3) of thetwo tunable filters 20 relative to ω₀ (of the microwave signal 305),such that one path (between ports 1 and 2) is in transmission while theother path (between ports 1 and 3) is in reflection, or vice versa.

To operate in reflection (i.e., block the microwave signal 305) foreither tunable filter 20 (between ports 1 and 2 or between ports 3 and4), one increases the DC currents I₂, I₃ to increase the magnetic biasflux Φ₂, Φ₃ (within 1 period of the cosine), which then increases theinductance L₂, L₃, thereby decreasing the cutoff angular frequencyω_(C2), ω_(C3). Conversely, to operate in transmission (i.e., pass themicrowave signal 305) for either tunable filter 20 (between ports 1 and2 or between ports 3 and 4), one decreases the DC currents I₂, I₃ todecrease the magnetic bias flux Φ₂, Φ₃ (within 1 period of the cosine),which then decreases the inductance L₂, L₃, thereby increasing thecutoff angular frequency ω_(Cz), ω_(C3).

The DC-SQUIDs 705, capacitors 50 (with the exception of the dielectricmaterial in the capacitors), flux lines 730, transmission lines 30, andJosephson junctions 710 are made of superconducting material. Again,examples of superconducting materials (at low temperatures, such asabout 10-100 millikelvin (mK), or about 4 K) include niobium, aluminum,tantalum, etc. A Josephson junction is a nonlinear element formed of twosuperconducting metals sandwiching a thin insulator such as, forexample, aluminum oxide, niobium oxide, etc.

FIG. 8 is a schematic of a superconducting microwave switch/router 100according to one or more embodiments. FIG. 8 is analogous to FIGS. 1-7,except for in this implementation, the tunable filters 20 are tunablehigh-pass filters. By having high-pass filters as the tunable filters20, the inductive elements 40, 705 are interchanged with the capacitiveelements 50. Accordingly, the capacitive elements 50 are in seriesbetween port 1 and 2 and between port 1 and 3, while the inductiveelements 40, 705 (inductor or DC-SQUID) connects to one end of thecapacitive element 50 and then connects to ground. For transmission fromport 1 to port 2 (or vice versa), the following condition appliesω_(C2)<ω₀<<ω_(C3). For transmission from port 1 to port 3 (vice versa),the following condition applies ω_(C3)<ω₀<<ω_(C2).

Now turning to FIG. 9, FIG. 9 is a schematic an N-port superconductingmicrowave router 100 according to one or more embodiments. The N-portsuperconducting microwave router 100 is generalized/designed such thatthere can be a connection made between any pair of ports 10 on the flyusing current pulses to the relevant flux lines which in turn flux biasthe relevant filters to their appropriate flux bias points. For example,at the moment (or nearly at the moment) the microwave signal 305 reachesa port 10, the connection can be made between any pair of ports 10 totransmit the microwave signal 305 while all other ports 10 (via theirrespective tunable filter 20) block the microwave signal 305.Accordingly, the microwave signal 305 can be routed between any pair ofports 10 (on the fly) according to the principles discussed herein.

The N-port superconducting microwave router 100 includes port 1, port I,port J, through port N. Each of the port 1-N has its own tunablelow-pass filter 20, such that an individual port 10 connects to atunable filter 20 that connects to a node 905. The features extensivelydescribed above in FIGS. 1-8 apply to FIG. 9 and are not repeated forthe sake of brevity and to avoid obscuring FIG. 9. All of the ports 1-Nare symmetrical and are on the same footing (which is different from thepreviously described superconducting microwave switches/router 100above). Being on the same footing means that the node 905 is a centralconnection that connects all of the ports 1-N, that each port 10 has itsown tunable filter 20, and that each tunable filter 20 has its own fluxline (FL) for tuning its cutoff frequency.

As one example, to route the microwave signal 305 from port N to port I,both tunable filters 20 between port N and node 905 and between port Iand node 905 have to be tuned to be in transmission; concurrently, allremaining tunable filters 20 are tuned to be in reflection. This allowsthe microwave signal 305 to be transmitted from port N to its tunablefilter 20, to the node 905, to tunable filter 20 connected to port I,and then transmitted to port I.

Regarding the node 905, a few technical details are discussed. Ingeneral, node 905 is to be as small as possible and ideally lumped withrespect to the wavelengths used in the device operation for tworeasons: 1) minimize reflections, which can limit the transmission ofthe routed signal, and 2) enable connecting multiple transmission linesto the node 905. Furthermore, the ability to connect multipletransmission lines to a common node 905 can require using high impedance(very narrow) wires, which might in turn require designing the tunablefilters to have a characteristic impedance which matches the impedanceof the connecting lines when the filters are operating in transmission(in order to minimize reflections) in one implementation. Lastly, if thecharacteristic impedance of the tunable filters is different from thecharacteristic impedance of the device ports, certain matching networkscan be designed and integrated between the filters and the device (inorder to allow smooth transmission for the propagating signals).

FIG. 10 is a flow chart 1000 of a method of configuring alossless/superconducting microwave switch/router 100 according to one ormore embodiments. Reference can be made to FIGS. 1-9 discussed herein.

At bock 1005, a plurality of ports 10 are provided. At block 1010,tunable filters 20 are provided and connected to the ports 10, such thateach of the plurality of ports 10 has a corresponding one of the tunablefilters 20.

The tunable filters 20 connect to a node 905 (a conductive connectionpoint). A plurality of flux lines (FL) 730 are provided, such that anindividual one of the plurality of flux lines 730 tunes an individualone of the tunable filters 20 on a one-to-one basis. A plurality ofmagnetic sources (such as flux lines, current carrying wires, tunablemagnets, etc.) are provided, such that an individual one of theplurality of magnetic sources tunes an individual one of the tunablefilters 20 on a one-to-one basis. It should be noted that this pictureof one flux line controlling one tunable filter can be simplistic. Thisis because the DC-SQUID's response/inductance is determined by the totalflux threading its loop, and therefore any change in the current ofother flux lines can alter, in principle, the flux bias experienced bythe DC-SQUID. Of course, the induced flux by the other flux lines dropsconsiderably with the distance between them and the DC-SQUID, thus bykeeping them sufficiently apart the experimenters can significantlyreduce their contribution. Nevertheless, there can be one or morescenarios that in order to tune the flux bias of one filter, one mightapply multiple changes to the currents flowing in nearby flux lines suchthat the currents yield the desired flux bias in the various controlledfilters.

The tunable filters 20 include superconducting material. Examplesuperconducting materials at superconducting temperatures (e.g., 10-100millikelvin (mK), or 4 K) can include niobium, aluminum, tantalum, etc.

The tunable filters 20 can be tunable lossless low-pass filters. Any oneof the plurality of ports 10 (e.g., port 1) is configured to transmit amicrowave signal 305 to any other one of the plurality of ports 10 (port2). The corresponding one of the tunable filters 20 for the any one ofthe plurality of ports 10 and the corresponding one of the tunablefilters 20 for the any other one of the plurality of ports 10 are bothconfigured to be tuned to transmit (i.e., in transmission) the signal305 while all other ones of the tunable filters 20 are configured to betuned to block the signal. Each of the lossless low-pass filtersincludes one or more DC SQUIDS in series with a center conductor of atransmission line and shunted by a capacitor to ground. It should beappreciated that a transmission line, such as a coaxial cable, has acenter conductor and an outer conductor.

FIG. 11 is a flow chart 1100 of a method for configuring alossless/superconducting microwave switch/router 100 according to one ormore embodiments. Reference can be made to FIGS. 1-9 discussed herein.

At block 1105, a plurality of ports 10 are provided. At block 1110,tunable filters 20 are connected to the plurality of ports 10, whereeach of the plurality of ports 10 is associated with one of the tunablefilters 20, where each of the tunable filters 20 includes asuperconducting quantum interference device 705. The tunable filters 20can be low-pass filters. The tunable filters 20 can be high-passfilters.

FIG. 12 is a flow chart 1200 of a method of configuring alossless/superconducting microwave switch/router 100 according to one ormore embodiments. Reference can be made to FIGS. 1-9 discussed herein.

At block 1205, a node 905 is provided as a central connection point. Atblock 1210, tunable filters 20 are connected to the node 905, where thetunable filters 20 are configured to be independently tuned to a firststate (i.e., mode of operation for transmission) to transmit a microwavesignal 305 and be independently tuned to a second state (i.e., mode ofoperation for reflection) to block the microwave signal 305, such thatany one of the tunable filters 20 is configured to transmit the signalto any other one of the tunable filters 20 via the node 905.

Any one of the tunable filters 20 and any other one of the tunablefilters 20 are both configured to be in the first state, while allremaining tunable filters 20 are configured to be in the second state,thereby allowing the microwave signal 305 to be transmitted from the anyone of the tunable filters 20 to the any other one of the tunablefilters 20 via the node 905.

FIG. 13 is a flow chart 1300 of a method of configuring alossless/superconducting microwave switch/router 100 according to one ormore embodiments. Reference can be made to FIGS. 1-9 discussed herein.

At block 1305, a plurality of ports 10 are provided. At block 1310, afirst pair of the plurality of ports 10 has at least one tunable filter20 connected in between, in which the tunable filter 20 is configured totransmit a microwave signal 305. At block 1315, a second pair of theplurality of ports 10 has another tunable filter 20 connected inbetween, in which the other tunable filter 20 is configured to reflectthe microwave signal.

Circulation of quantum signals plays an important role in quantuminformation processing. It allows operators to separate between incomingand outgoing signals propagating on the same transmission line. This isparticularly useful when measuring a device (such as a qubit) inreflection.

State-of-the-art commercial cryogenic circulators are bulky (therebylimiting scalability), have finite loss on the order of 0.5-1 dB(thereby causing some of the quantum information to be lost), and usestrong magnetic fields that can interfere with superconducting circuits.Also, state-of-the-art circulators use ferrites, which are magneticmaterials that cannot be easily integrated on chip.

For all these reasons, there is need for on-chip lossless circulators.Embodiments provide on-chip lossless circulators by utilizing thelossless/superconducting microwave switch/router 100 as a circulator.Routing of quantum signals is expected to play a particular role inscalable quantum processor architectures. Such capability allowsoperators to significantly decrease the number of input and outputlines, and the amount of hardware/components which are used in order todrive, couple, and read out qubits. FIG. 14 is a simplified view of thelossless/superconducting microwave switch/router 100 for quantum signalsthat can connect between arbitrary pairs of nodes in-situ (e.g., at anyselected/desired moment in time) according to one or more embodiments.The lossless/superconducting microwave switch/router 100 includestunable filters 20 which operate as a switch that can be closed (i.e.,transmission) or open (i.e., reflection) as discussed herein.Accordingly, the lossless/superconducting microwave switch/router 100can be considered as having two modes of operation. In the first mode,all nodes/ports are reflective at the working frequency. In the secondmode, two nodes/ports are connected together, and quantum signals at theworking frequency can be transmitted between the two nodes/ports withunity transmission, while all other nodes/ports are isolated.

FIGS. 15A, 15B, 15C, and 15D illustrate circulation between 3 portsutilizing the lossless/superconducting microwave switch/router 100 as acirculator according to one or more embodiments.

FIG. 15A depicts the lossless/superconducting microwave switch/router100 with all tunable filters 20 in reflection. Accordingly, each tunablefilter 20 functions as an open switch at this point in time, such thatno quantum signal is allowed to pass.

FIG. 15B depicts the lossless/superconducting microwave switch/router100 in a configuration that permits transmission of quantum signalsbetween two ports according to one or more examples. In FIG. 15B, thetransmission is between ports 2 and 3 (in both directions) at this pointin time. The quantum signal is a microwave signal. The microwave signal305 is designated as 305A and 305B to illustrated bi-directionaltransmission. Accordingly, example quantum signals 305A and 305B can betransmitted between ports 2 and 3 because the tunable filter 20connected to port 3 functions as a closed switch (i.e., in transmission)and the tunable filter 20 connected to port 2 functions as a closedswitch (i.e., in transmission), all while the tunable filter 20connected to port 1 functions as an open switch (i.e., in reflection).

FIG. 15C depicts the lossless/superconducting microwave switch/router100 in a configuration that permits transmission of quantum signalsbetween two ports according to one or more examples. In FIG. 15C, thetransmission is between ports 1 and 2 (in both directions) at this pointin time. Accordingly, quantum signals 305A and 305B can be transmittedbetween ports 1 and 2 because the tunable filter 20 connected to port 1functions as a closed switch (i.e., in transmission) and the tunablefilter 20 connected to port 2 functions as a closed switch (i.e., intransmission), all while the tunable filter 20 connected to port 3functions as an open switch (i.e., in reflection).

FIG. 15D depicts the lossless/superconducting microwave switch/router100 in a configuration that permits transmission of quantum signalsbetween two ports according to one or more examples. In FIG. 15D, thetransmission is between ports 1 and 3 (in both directions) at this pointin time. Accordingly, quantum signals 305A and 305B can be transmittedbetween ports 1 and 3 because the tunable filter 20 connected to port 1functions as a closed switch (i.e., in transmission) and the tunablefilter 20 connected to port 3 functions as a closed switch (i.e., intransmission), all while the tunable filter 20 connected to port 2functions as an open switch (i.e., in reflection).

As can be seen in FIGS. 15A, 15B, 15C, 15D, the lossless/superconductingmicrowave switch/router 100 can be configured to operate as a circulatorthat permits signals to be passed from one port to another port whileisolating a third port. Although 3 ports are shown for illustration, itshould be appreciated that the concepts for operating the circulatorapplies to more than 3 ports.

To further illustrate operation as a circulator, FIGS. 16A, 16B, 16C,and 16D depict time-dependent partial circulation according to one ormore embodiments. FIGS. 16A, 16B, 16C, and 16D illustrate separationbetween input and output signals of a resonator-qubit system. In FIGS.16A, 16B, 16C, and 16D, the lossless/superconducting microwaveswitch/router 100 has an IN signal port designated as port 1, an OUTsignal port designated as port 2, and port 3. Port 3 is connected to aquantum system 1650. The quantum system 1650 is a resonator 1620 andqubit 1615 and can be referred to as a resonator-qubit system. The qubit1615 and resonator 1620 can be capacitively connected, can be connectedin a two-dimensional cavity, and/or can be connected in athree-dimensional cavity as understood by one skilled in the art. Theport 1 has its own tunable filter 20 and the port 2 has its own tunablefilter 20. However, port 3 does not have its own tunable filter and iscontinually connected to the common node 905 of thelossless/superconducting microwave switch/router 100.

FIGS. 16A, 16B, 16C, and 16D show a delay line (or delay buffer) 1605connected to port 3 of the lossless/superconducting microwaveswitch/router 100. A coupling capacitor 1610 connects the delay line1605 to the resonator 1620 of the quantum system 1650.

FIG. 16A depicts an example scenario at time t₁ according to anembodiment. At time t₁, the tunable filter 20 connected to input port 1is functioning as a closed switch (i.e., in transmission). A readoutsignal 305 is received on port 1. The readout signal 305 is a readoutpulse with frequency f_(r) and pulse time duration T_(p). The frequencyf_(r) is the resonance frequency of the readout resonator 1620. Thereadout resonator 1620 is designed to readout the state of the qubit1615. The readout resonator 1620 has a resonator bandwidth K. The delayline/buffer 1605 has a time delay duration of T_(d). The time delay andreadout pulse durations have the relationship of T_(d)>T_(p).

At time t₁, the readout signal 305 is input on the IN port 1,transmitted on transmission line 30, and transmitted to port 3. At timet₁, the tunable filter 20 on port 2 is functioning as an open switch,such that the readout signal 305 cannot exit the OUT port 2.

Continuing forward in time, FIG. 16B depicts an example scenario at timet₂ according to an embodiment. At time t2, the readout signal 305 is nowbeing delayed by the delay line/buffer 1605 for a time delay duration ofT_(d), before being transmitted to the readout resonator 1620. At thispoint, the tunable filter 20 connected to the IN port 1 is still closed(i.e., the input switch is close). The time delay T_(d) provides timefor the tunable filter 20 on port 2 to switch to close (i.e., operate asa closed switch) before the return readout signal is transmitted fromthe readout resonator 1620, and also provides time for the tunablefilter 20 on port 1 to switch to open (i.e., operate as an open switch).Because T_(d)>T_(p), the pulse of the readout signal 305 is allowed tocomplete transmission on IN port 1 before any reflected readout signalreturns from the readout resonator 1620.

Still continuing forward in time, FIG. 16C depicts an example scenarioat time t₃ according to an embodiment. At time t₃, the readout signal305 has now reached the readout resonator 1620 after the time delayT_(d) by the delay line/buffer 1605, and the reflected readout signal305′ (e.g., the return readout signal) is beginning to betransmitted/reflected from the readout resonator 1620. The reflectedreadout signal 305′ is likewise delayed by the delay line/buffer 1605for the time delay duration of T_(d), before being transmitted to OUTport 2. At this point, the tunable filter 20 connected to the IN port 1is open (i.e., the input switch is open), and the tunable filter 20connected to the OUT port 2 is closed (i.e., the output switch isclosed). The time delay T_(d) has provided transition time for thetunable filter 20 (connected to IN port 1) to switch to open such thatthe return readout signal 305′ cannot be transmitted to IN port 1.

Further continuing forward in time, FIG. 16D depicts an example scenarioat time t₄ according to an embodiment. At time t₄, the return/reflectedreadout signal 305′ has now reached the OUT port 2. The reflectedreadout signal 305′ is transmitted from the readout resonator 1620 withstate information of the qubit 1615, travels through the delayline/buffer 1605, travels through port 3, and exits OUT port 2. At timet₄, the tunable filter 20 (connected to the IN port 1) is functioning asan open switch such that the reflected readout signal 305′ cannot exitthe IN port 1. The transition time provided by the delay line/buffer1605 ensures that none of the reflected readout signal 305′ is able toexit the IN port 1 while the tunable filter 20 (connected to the IN port1) is in transition (changing to an open switch from a closed switch).In one implementation, the delay line/buffer 1605 can be a transmissionline with a length suitable to delay the readout signal 305 and thereturn readout signal 305′ a time delay duration T_(d). For an examplereadout pulse duration of 80 nanoseconds, the delay line/buffer 1605 canbe a transmission line of a few meters fabricated on a small chip in avery compact manner. The scenario provided in FIGS. 16A, 16B, 16C, and16D illustrates a readout signal 305 input through the IN port 1, withthe reflected readout signal 305′ returned through the OUT port 2, suchthat the IN port 1 is isolated from the OUT port 2. The reflectedreadout signal 305′ returns with quantum information about the qubit1615, as understood by one skilled in the art.

As discussed above in FIGS. 14, 15A-D, 16A-D, thelossless/superconducting microwave switch/router 100 is a circulator forrouting quantum signals in the microwave domain using time-dependentswitching. Some details are not shown in FIGS. 14, 15A-D, 16A-D so asnot to obscure the figures, but are understood to be present. Theinternal details of the circulator (lossless/superconducting microwaveswitch/router 100) in FIGS. 15A-D and 16A-D are analogous to thedescription in FIGS. 5, 6, 7, and 8 discussed herein. In particular, 3ports are shown and only 2 ports have their own tunable filter 20. FIG.14 is analogous to FIG. 9 where each port has its own tunable filter 20.

It is noted that FIGS. 14, 15A-D, 16A-D do not show the individual fluxlines 730 in proximity to the tunable filters 20, where the flux lines730 are control lines which are controlling the tunable filters 20 tofunction as open and closed switches. The current on the flux lines 730are time-dependent control signals that determine when to open (operatein reflection) and close (operate in transmission) the tunable filters20. For example, a controller 1660 is shown in FIGS. 14, 15A-D, 16A-D,and the controller 1660 is configured to apply individual controlsignals (i.e., current) to respective flux lines 730 that tune therespective tunable filters 20, such that the tunable filters 20 operateas an open switch or closed switch as controlled by the controller 1660.

Taking FIGS. 16A-16D as an example, the controller 1660 is configured tocause the tunable filter 20 connected to the IN port 1 to operate as aclosed switch at times t₁ and t₂, and to operate as an open switch attimes t₃ and t₄. The controller 1660 is configured to cause the tunablefilter 20 connected to the OUT port 2 to operate as an open switch attimes t₁ and t₂, and to operate as a closed switch at times t₃ and t₄.The controller 1660 is connected to and controls the current flowing onthe flux line 730 associated with IN port 1 and the flux line 730associated with OUT port 2. In one implementation, the controller 1660can include a processor (or microcontroller), memory, a timingmechanism, etc. The controller 1660 is configured to executeinstructions in the memory, and the instructions control the timing ofthe control signals that control the operation (e.g., open switch orclosed switch) of the tunable filters 20. The controller 1660 isconfigured to control one or more voltage sources, current sources,and/or power supplies that supply the current to the flux lines 730. Thecontroller 1660 can be a computer.

In one implementation, the controller 1660 can be or include an analogdevice. The controller 1660 can include timed switches and/or relaysthat apply the desired amount of current on the flux lines 730 such thatthe tunable filters 20 are tuned as desired.

It should be appreciated that the controller 1660 can tune the tunablefilters 20 to operate as an open switch and closed switch according to apredefined timing pattern.

FIG. 17 is a flow chart 1700 of a method of configuring asuperconducting router 100 according to one or more embodiments.Reference can be made to FIGS. 1-14, 15A-D, and 16A-D.

At block 1705, the superconducting router 100 is configured to operate(e.g., via control signals from the controller 1660) in a first mode(e.g., according to a predefined time or times), in which all ports 10are configured to be in reflection in the first mode in order to reflecta signal 305.

At block 1710, the superconducting router 100 is configured to operate(e.g., via control signals from the controller 1660) in a second mode(e.g., according to another predefined time or times), in which a givenpair (e.g., ports 2 and 5 in FIG. 14) of the ports 10 are connectedtogether and in transmission in a second mode, such that the signal 305is permitted to pass between the given pair of the ports 10.

The ports 10 are in reflection in the first mode for a predefinedfrequency of the signal 305. The given pair of the ports is intransmission in the second mode for a predefined frequency.

The ports 10 are configured to be isolated from one another, such thatany port 10 is controllable to be separated from another port 10 bycontrolling the respective tunable filters 20.

The superconducting router 100 includes superconducting materials. Therouter 100 includes tunable filters 20 associated with the ports 10,such that the tunable filters are controllable to operate in the firstmode and the second mode as desired. The tunable filters 20 areconfigured to be operated as an open switch and a closed switchaccording to predefined requirements in the first and second modes. Inthe second mode, the given pair of the ports is in transmission whileother ports of the ports are in reflection for a predefined frequency ofthe signal.

The signal 305 (including 305A, 305B, and 305′) is in a microwavedomain. The given pair of the ports is time dependent, such that aselection of the given pair of the ports is configured to changeaccording to a predefined time scheme. The given pair of the ports isconfigured to be arbitrarily selected from the ports. The given pair ofthe ports is configured to be selected from the ports at a predefinedtime. The router is a lossless microwave switch 100 havingsuperconducting materials.

FIG. 18 is a flow chart 1800 of a method of configuring asuperconducting circulator 100. Reference can be made to FIGS. 1-14,15A-D, 16A-D, and 17.

At block 1805, the superconducting circulator 100 is configured tooperate (e.g., via control signals from the controller 1660) to receivea readout signal 305 at an input port (e.g., port 1) while an outputport is in reflection (e.g., as depicted in FIG. 16A). The readoutsignal is to be transmitted through a common port (e.g., port 3) to aquantum system 1650, e.g., as depicted in FIG. 16B. The readout signal305 is configured to cause a reflected readout signal 305′ to resonateback from the quantum system 1650, e.g., as depicted in FIG. 16C.

At block 1810, the superconducting circulator 100 is configured tooperate to output the reflected readout signal 305′ at the output port(e.g., port 2) while the input port (e.g., port 3) is in reflection, asdepicted in FIG. 16D.

A delay line 1605 delays transmission of the readout signal 305 and thereflected readout signal 305′. A transition time (e.g., delay timeT_(d)) is provided to switch between operating the input port intransmission to operating the input port in reflection, and the delayline 1605 causes the transition time. The quantum system 1650 includes areadout resonator 1620 operatively connected to a qubit 1615. Thereflected readout signal 305′ includes quantum information of the qubit1615.

A first tunable filter 20 is connected to the input port (e.g., port 1),such that the first tunable filter 20 permits the input port to operatein transmission or reflection. Similarly, a second tunable filter 20 isconnected to the output port (e.g., port 2), such that the secondtunable filter 20 permits the output port to operate in transmission orreflection. The superconducting circulator 100 is a lossless microwaveswitch having superconducting materials.

Technical effects and benefits include a lossless/superconductingmicrowave switch/router. Technical benefits further include lowattenuation of transmitted signals <0.05 dB (decibels), fast switching(no resonators) such as in nanoseconds (depending on the mutualinductance between the flux lines and the SQUIDs), and relatively largebandwidth (bw) >280 megahertz (MHz) (which can be significantly enhancedby allowing certain variation in the unit cells). Also, technicalbenefits further include relatively large on/off ratio >20 dB. Thelossless/superconducting microwave switch/router can tolerate relativelylarge powers >−80 dBm (where 0 dBm corresponds to 1 milliwatt) by addingmore SQUIDs and increasing their critical current. Thelossless/superconducting microwave switch/router can be fabricated withNb Josephson junctions to operate at 4K, can be designed for anyfrequency range, and provides a scalable scheme that can be extended to1 input-N outputs (or vice versa).

The term “about” and variations thereof are intended to include thedegree of error associated with measurement of the particular quantitybased upon the equipment available at the time of filing theapplication. For example, “about” can include a range of ±8% or 5%, or2% of a given value.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams can represent a module, segment, or portionof instructions, which includes one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block can occur out of theorder noted in the figures. For example, two blocks shown in successioncan, in fact, be executed substantially concurrently, or the blocks cansometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments described herein. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method of configuring a superconducting router,the method comprising: operating the superconducting router in a firstmode, wherein ports are configured to be in reflection in the first modein order to reflect a signal; and operating the superconducting routerin a second mode, wherein a given pair of the ports is coupled togetherand in transmission in the second mode, such that the signal ispermitted to pass between the given pair of the ports, one of the givenpair of the ports having a single tunable filter operating intransmission for the one of the given pair, wherein operating intransmission for the one of the given pair relies on operation of thesingle tunable filter and is independent from operation of other tunablefilters having the ports in reflection, the single tunable filtercomprising a unit cell void of a hybrid coupler.
 2. The method of claim1, wherein the ports are in reflection in the first mode for apredefined frequency of the signal.
 3. The method of claim 1, whereinthe given pair of the ports are in transmission in the second mode for apredefined frequency.
 4. The method of claim 1, wherein the ports areconfigured to be isolated from one another, such that any port iscontrollable to be separated from another port.
 5. The method of claim1, wherein the superconducting router comprises superconductingmaterials.
 6. The method of claim 1, wherein the superconducting routercomprises the other tunable filters which are individually associatedwith the ports, such that the other tunable filters and the singletunable filter are controllable to operate in the first mode and thesecond mode.
 7. The method of claim 1, wherein the other tunable filtersand the single tunable filter are configured to be independentlyoperated as an open switch and a closed switch according to the firstand second modes.
 8. The method of claim 1, wherein in the second mode,the given pair of the ports is in transmission while other ones of theports are in reflection for a predefined frequency of the signal.
 9. Themethod of claim 1, wherein the signal is in a microwave domain.
 10. Themethod of claim 1, wherein the given pair of the ports are timedependent, such that a selection of the given pair of the ports areconfigured to change according to a defined time scheme.
 11. The methodof claim 1, wherein the given pair of the ports are configured to bearbitrarily selected from the ports.
 12. The method of claim 1, whereinthe given pair of the ports are configured to be selected from the portsat a defined time.
 13. The method of claim 1, wherein thesuperconducting router is a lossless microwave switch havingsuperconducting materials.
 14. A method of configuring a superconductingcirculator, the method comprising: operating the superconductingcirculator to receive a readout signal at an input port while an outputport is in reflection and while a first tunable filter coupled to theinput port is in transmission as a second tunable filter coupled to theoutput port is in reflection, wherein the readout signal is to betransmitted through a common port to a quantum system, wherein thereadout signal is configured to cause a reflected readout signal toresonate back from the quantum system, the first tunable filtercomprising a unit cell void of a hybrid coupler; and operating thesuperconducting circulator to output the reflected readout signal at theoutput port while the input port is in reflection.
 15. The method ofclaim 14, wherein a delay line delays transmission of the readout signaland the reflected readout signal.
 16. The method of claim 15, furthercomprising providing a transition time to switch between operating theinput port in transmission to operating the input port in reflection,wherein the delay line causes the transition time.
 17. The method ofclaim 14, wherein the quantum system includes a readout resonatoroperatively connected to a qubit.
 18. The method of claim 17, whereinthe reflected readout signal includes quantum information of the qubit.19. The method of claim 14, wherein the first tunable filter permits theinput port to operate in transmission or reflection; and wherein thesecond tunable filter permits the output port to operate in transmissionor reflection.
 20. The method of claim 14, wherein the circulator is alossless microwave switch having superconducting materials.